The present invention relates to an electronic thermal relay which can be associated with a multiphase load for cutting off the power supply to this load when a current overload in at least one phase or an imbalance between the phase currents occurs. It also relates to a thermal element appropriate for such a relay.
The mechanical thermal relays usually used for protecting multiphase motors comprise a bimetallic strip per phase and a separable contact assembly. The bimetallic strips have all or part of the phase currents flowing therethrough and, when an overload occurs, the bimetallic strip of the overloaded phase or phases deflects and acts mechanically on a tripping mechanism, which causes opening of the contact assembly.
These known relays are very robust and of a low cost price. But, among other drawbacks, they do not provide sufficient precision of the tripping threshold under permanent operating conditions, they do not allow an alarm prior to tripping and they do not satisfactorily process hot starting of the motors with which they are associated.
To protect multiphase motors, electronic relays are also used. These relays comprise for each phase a detecting means such as a current transformer delivering an image signal of the intensity of the current in the phase, as well as an electronic processing circuit which receives and processes the signals delivered by the detecting means and delivers a tripping signal. These relays provide a pre-alarm and good accuracy under permanent operating conditions, but they are complicated which generates an increase in cost, size and the likelihood of defects. Furthermore, they only correctly process hot starts by memorizing the thermal condition of the motor which generates the above-mentioned drawbacks.